Re: [Qemu-devel] QEMU -netdev vhost=on + -device virtio-net-pci bug

[Alexey Kardashevskiy <aik@ozlabs.ru>]

Michael,

Thanks for the fix.

There was another question which was lost in the thread.

I am testing virtio-net in two ways:

Old -net interface:
-net tap,ifname=tap0,script=qemu-ifup.sh \
-net nic,model=virtio,addr=0:0:0

(qemu) info network
hub 0
  \ virtio-net-pci.0: 
index=0,type=nic,model=virtio-net-pci,macaddr=52:54:00:12:34:56
  \ tap.0: 
index=0,type=tap,ifname=tap0,script=qemu-ifup.sh,downscript=/etc/qemu-ifdown

New -netdev interface:
-netdev tap,id=tapnet,ifname=tap0,script=qemu-ifup.sh \
-device virtio-net-pci,netdev=tapnet,addr=0.0

(qemu) info network
virtio-net-pci.0: 
index=0,type=nic,model=virtio-net-pci,macaddr=52:54:00:12:34:56
  \ tapnet: 
index=0,type=tap,ifname=tap0,script=qemu-ifup.sh,downscript=/etc/qemu-ifdown


I get very different virtio0 device features and speed (70MB/s vs. 
700MB/s). I guess somehow the "hub 0" is responsible but there is no way to 
avoid it.

Is there any way to speed up the virtio-net using the old -net interface?



On 06/03/13 21:31, Michael S. Tsirkin wrote:
> On Wed, Mar 06, 2013 at 09:57:40AM +1100, Alexey Kardashevskiy wrote:
>> On 06/03/13 01:23, Michael S. Tsirkin wrote:
>>> On Wed, Mar 06, 2013 at 12:21:47AM +1100, Alexey Kardashevskiy wrote:
>>>> On 05/03/13 23:56, Michael S. Tsirkin wrote:
>>>>>> The patch f56a12475ff1b8aa61210d08522c3c8aaf0e2648 "vhost: backend
>>>>>> masking support" breaks virtio-net + vhost=on on PPC64 platform.
>>>>>>
>>>>>> The problem command line is:
>>>>>> 1) -netdev tap,id=tapnet,ifname=tap0,script=qemu-ifup.sh,vhost=on \
>>>>>> -device virtio-net-pci,netdev=tapnet,addr=0.0 \
>>>>>
>>>>> I think the issue is irqfd in not supported on kvm ppc.
>>>>
>>>> How can I make sure this is the case? Some work has been done there
>>>> recently but midnight is quite late to figure this out :)
>>>
>>> Look in virtio_pci_set_guest_notifiers, what is the
>>> value of with_irqfd?
>>>    bool with_irqfd = msix_enabled(&proxy->pci_dev) &&
>>>          kvm_msi_via_irqfd_enabled();
>>>
>>> Also check what each of the values in the expression above is.
>>
>> Yes, ppc does not have irqfd as kvm_msi_via_irqfd_enabled() returned "false".
>>
>>>>> Could you please check this:
>>>>>
>>>>> +        /* If guest supports masking, set up irqfd now.
>>>>> +         * Otherwise, delay until unmasked in the frontend.
>>>>> +         */
>>>>> +        if (proxy->vdev->guest_notifier_mask) {
>>>>> +            ret = kvm_virtio_pci_irqfd_use(proxy, queue_no, vector);
>>>>> +            if (ret < 0) {
>>>>> +                kvm_virtio_pci_vq_vector_release(proxy, vector);
>>>>> +                goto undo;
>>>>> +            }
>>>>> +        }
>>>>>
>>>>>
>>>>> Could you please add a printf before "undo" and check whether the
>>>>> error path above is triggered?
>>>>
>>>>
>>>> Checked, it is not triggered.
>>>>
>>>>
>>>> --
>>>> Alexey
>>>
>>> I think I get it.
>>> Does the following help (probably not the right thing to do, but just
>>> for testing):
>>
>>
>> It did not compile (no "queue_no") :) I changed it a bit and now
>> vhost=on works fine:
>>
>> diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c
>> index a869f53..df1e443 100644
>> --- a/hw/virtio-pci.c
>> +++ b/hw/virtio-pci.c
>> @@ -798,6 +798,10 @@ static int
>> virtio_pci_set_guest_notifiers(DeviceState *d, int nvqs, bool
>> assign)
>>           if (r < 0) {
>>               goto assign_error;
>>           }
>> +
>> +        if (!with_irqfd && proxy->vdev->guest_notifier_mask) {
>> +            proxy->vdev->guest_notifier_mask(proxy->vdev, n, !assign);
>> +        }
>>       }
>>
>>       /* Must set vector notifier after guest notifier has been assigned */
>>
>>
>
> I see, OK, the issue is that vhost now starts in a masked state
> and no one unmasks it. While the patch will work I think,
> it does not benefit from backend masking, the right thing
> to do is to add mask notifiers, like what the irqfd path does.
>
> Will look into this, thanks.
>


-- 
Alexey